Patent GB-418442-A (GB)
Title: Improvements in or relating to electric signalling systems
IPC: H04Q1/446
Filed: 1933-04-20
Abstract: 418,442. Valve circuits. RYALL, L. E., 52, Crescent Gardens, Eastcote, Ruislip, Middlesex. April 20, 1933, No. 11571. [Class 40 (v).] In an alternating current signal receiver which discriminates against an interfering signal, the receiving relay is operated by the A.C. component of anode current of a thermionic valve in the transmission path, the D.C. component being considerably varied by an interfering signal and being used to control an attenuating network in the transmission path so as to prevent operation of the receiver by the interfering signal. The receiver is applied in a telephone system to respond to a signal of definite frequency but not to respond to speech. The discrimination may be effected by the large peaks or various frequencies present in speech. The attenuation may be increased automatically by excessive levels of desired signals. Discrimination against speech currents due to the presence of large peak amplitudes therein is used in the arrangement shown in Fig. 2. Incoming currents pass transformer 1 to an attenuating network comprising rectifiers 12, 13 in series and shunt as described in Specification 413,383 and are applied to valve 4 the output circuit of which comprises a transformer 8 tuned to the signal frequency (say 500 cycles) and a relay 10 fed by rectifiers 9. Normally, the opposed voltages over resistance 7 in the anode circuit and resistance 14 across the filament battery are such that 17 is positive with respect to 16 and current flows through the rectifiers 12, 13 in such a direction that their A.C. impedance is low and high respectively so that the attenuation of the network is low. The grid circuit of the valve has a condenser leak 5, 6, and on the arrival of a signal, anode current decreases thus lowering the potential at 17 so that the attenuation of the network increases, but not to an extent seriously affecting the signal. Relay 10 therefore pulls up. When, however, speech currents arrive, the peaks are greater than the amplitude of the signal, and anode current is depressed sufficiently to reverse the current flow through the rectifiers 12, 13 whereby the attenuation increases to such an extent that relay 10 is not operated. By suitably choosing leak resistance 6 sufficient charge on condenser 5 is maintained between the peaks to prevent operation of relay 10. Negative bias may be applied to the grid by taking incoming currents from an additional winding of the input transformer 1 and rectifying them. The relay 10 may also be inserted in the grid circuit. In the modified arrangement shown in Fig. 2, frequency discrimination is also used, the circuits (not shown) for controlling the attenuation network being the same as in Fig. 1. The peaks of speech operate as before but, in addition, speech 'voltages built up across a circuit 19 tuned to and having low impedance at the signal frequency, are rectified at 20 and applied across resistance 6 to increase the discrimination against speech. To prevent low frequency oscillation, the grid of valve 4 is connected to a centre tap on the secondary of transformer 3 the ends of which are connected to the cathode circuit over equal resistances 23, a byepass condenser 25 being provided. In the two-frequency receiver shown in Fig. 3, the receiving relays 26 are operated by valves 27 coupled in parallel to the output circuit of 4, the primaries of transformers 28 being tuned to the desired frequencies. If the level of the incoming signals is excessive, part of the input to transformer 28 rectified at 31 and applied to resistance 6 reduces the anode current of valve 4 to increase the attenuation of the input network as in Figs. 1 and 2.